1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD), and more particularly to a liquid crystal display device in which an additive is added to a photo-alignment material for forming a photo-alignment film of an LCD so that photo-stability can be enhanced and image sticking can be improved.
2. Discussion of the Related Art
Referring to FIG. 1, for example, an LCD generally includes transparent substrates 11 and 12 that are opposed to each other by a spacer (not shown) for maintaining a cell gap with a distance that allows injection of a liquid crystal 13 which is sealed by a sealant 14.
The transparent substrate 11 is provided with a plurality of pixel electrodes 15 covered with an alignment film at the inner surface. Each of the pixel electrodes 15 is provided with a thin film transistor (TFT) 16 which functions as a switching device. Here, a drain electrode of a TFT 16 is connected with each pixel electrode 15.
Meanwhile, the other transparent substrate 12 is provided at the inner surface with a transparent common electrode 17 opposed to the plurality of pixel electrodes 15 and covered with another alignment film.
FIG. 2 shows the above-described LCD together with a driving circuit thereof.
The LCD comprises a liquid crystal panel 20, a scanning line driving circuit 21 for driving the liquid crystal panel 20 and a signal line driving circuit 22.
A plurality of scanning lines 23 and a plurality of signal lines 24 are placed on a substrate of the liquid crystal panel 20 such that they intersect each other in the pattern of a matrix, and the thin film transistor 16 and the pixel electrode are installed at one of the intersecting portions thereof.
The scanning line driving circuit 21 transmits scanning signals, which transmit ON signals to the gate of the thin film transistor 16 and to the scanning lines 23 in sequence. The signal line driving circuit 22 transmits image signals to the signal lines 24 so that the image signals can be transferred to the pixel through the thin film transistor 16 driven by the scanning signals.
When the scanning line driving circuit 21 transmits the scanning signals in sequence to the scanning lines 23 of the liquid crystal panel 20 so that all of the thin film transistors 16 connected to the scanning lines 23 are powered on or energized, the signals applied to the signal lines 24 of the liquid crystal panel 20 are transferred to the pixels through sources and drains of the thin film transistors 16.
According to the aforementioned operation principle, the pulse is transferred to all gate electrodes in sequence and a signal voltage is applied to a corresponding source electrode so that all the pixels of the liquid crystal panel can be driven. After an image of one frame is displayed in this manner, the next frames are continuously displayed to achieve a dynamic image display.
In such image display, a vast amount of information such as a color display cannot be expressed by driving only white and black pixels. Therefore, a gradation (gray) display is implemented, in which several intermediate states further exist between white and black states. Referring to a black-and-white LCD, when an intermediate voltage is applied, an intermediate state such as a gray color exists to display information.
In order to obtain an intermediate value of voltage, the voltage intensity is adjusted, or the width of voltage pulse is adjusted.
In a color LCD, a color display is determined based on the degree of the gradation display.
A driving IC of 6 bits can produce 64 gradations, and a monitor or audio/video (AV) product that requires a full color spectrum has 16,000,000 colors in 256 gradations.
As in the foregoing description, since an LCD is an apparatus that adjusts the magnitude of the voltage applied to the liquid crystal to display information on a screen, the gradation of an LCD is adjusted based the degree of light transmission varying according to voltage.
The LCD is generally comprised of two substrates oppositely arranged with a predetermined distance from each other and the liquid crystal is injected between the two substrates.
In the case of a TFT color LCD, the first substrate is provided with pixel electrodes, transistors for driving pixels and an alignment film. The second substrate is provided with another alignment film, RGB color filters and a common electrode, and the liquid crystal is injected between the two substrates to complete the construction of the LCD.
However, as is well known in the art, since the liquid crystal has refractive anisotropy about the short and long axes, the LCD requires that the arrangement of liquid crystal molecules be uniformly controlled in order to obtain uniform brightness and high contrast ratio. For this purpose, the substrate surface defining a liquid crystal cell is coated with the alignment film, and an orientation treatment such as rubbing is performed to impart an orientation to the liquid crystal.
An LCD using the photo-alignment film of the related art has several drawbacks. For example, the easy axis is rotated upon exposure to external light to lower luminance, light-leakage takes place and, in particular, the contrast ratio is lowered.